The present invention relates to an digital watermark technique and a digital watermark detection method for digital data especially image data.
From the standpoint of protection of a copyright to works such as image data and music (hereinafter referred to as contents), the digital watermark technique has been noticed. The digital watermark technique is a technique of inserting predetermined information (watermark information) into digital data (hereinafter simply referred to as data) by directly changing a data value at a predetermined position through the use of a predetermined rule. Taking image data, for instance, information concerning a purchaser of the image data is inserted in invisible form into the image data at a predetermined position thereof in accordance with a predetermined rule and in the event that the image data is copied without authorization, the inserted predetermined information is detected from the data copied without authorization by using a pixel value at the predetermined position in accordance with the predetermined rule to specify a person who committed the unauthorized copying.
The digital watermark technique is described in “General Quality Maintenance Module for Motion Picture Watermarking” by I. Echizen, H. Yoshiura, T. Arai, H. Kimura and T. Takeuchi, IEEE Transactions on Consumer Electronics, Vol. 45, No. 4, pp. 1150–1158, November 1999.
The image data has possibly been applied with various image processes of geometrical conversion such as enlargement, reduction, compression and expansion and of trimming in the course of usage and circulation after creation of the image data.
When image format conversion or geometrical transformation such as enlargement/reduction and rotation is applied to the image data having watermark information inserted in this manner, that is, when transformation is applied to the digital data, the pixel value at a predetermined position that has been changed by the watermark insertion is also changed by the transformation. This leads to a failure to detect the watermark. As an example of technique to solve this problem, a technique described in U.S. Pat. No. 5,636,292 is available.
Further, as described in “Techniques for data hiding” by W. Bender, D. Gruhl and N. Morimoto, Proc. SPIE, Vol. 2420, pp. 164–173 (1995), a method for embedding information of one bit by operating several thousands of pixels is known. In this method (“digital watermark utilizing statistical properties”), an artificial pixel operation is detected statistically by utilizing properties of a natural image. Therefore, during detection of watermark information, there needs no original image data prevailing before embedding of a watermark. In this watermark method, even when transformation or trimming of data is carried out, the watermark information is relatively difficult to lose.
On the other hand, there are many image formats of input image data representing an object to be embedded with information. As factors for determination of an image format, the number of pixels of the image, a color expression method (GRB type, YUV type, YCrCb type, CMYK type or the like) and a color gradation number are available and as factors for determination of a format of a motion picture, the number of lines in the horizontal direction of an image, an interlace method or a progressive method is available.
There is a possibility that the image data embedded with digital watermark information is further applied with various image processes such as JPEG compression, MPEG compression and geometrical transformation. “A Secure, Robust Watermark for Multimedia” by I. Cox, J. Kilian, T. Leighton and T. Shamoon, Proceedings of Information Hiding (LNCS 1174), pp. 185–206 (1996) describes a method of embedding watermarks in MPEG compressed images.
Thus, there are many image formats that need to be presupposed in a detection process. Accordingly, if all image formats or transformation patterns are to be dealt with in the digital watermark information detection process, detection algorithm becomes sophisticated and the time required for detection increases. Further, it is necessary to deal with an image having the maximum number of pixels defined by the image format, so that the memory capacity increases to increase the hardware size of the detection processor.
Actual distribution of the image data will now be considered. FIG. 4 is an apparatus construction diagram showing an example of format conversion in motion picture data between units. A DVD-ROM 401 stores watermarked motion picture data. A watermark is embedded in a format of the motion picture data, in which format the size of each frame image is defined by 720 pixels in the vertical direction and 480 pixels in the horizontal direction ((720×480) pixels). A DVD player 402 for reproducing the DVD-ROM 401 displays a motion picture on a motion picture reproduction monitor 403 through an analog output. The motion picture reproduction monitor 403 has format display conversion function 404 by which several kinds of formats can be displayed. The display conversion 404 applies a format conversion to inputted motion picture data and thereafter displays the transformed motion picture data on the screen and delivers an analog output. For example, the display conversion 404 converts watermarked motion picture data having a format of 720×480 pixels into a format of 360×240 pixels and a DVD recorder 405 records the motion picture data on a DVD-RAM 406 through an analog output. The image stored in the DVD-RAM 406 is watermarked motion picture data in which the number of pixels in the vertical and horizontal directions is ½ reduced (at a reduction ratio of 1/4). In the apparatus construction as above, the transformation applied to the image data is defined by only several kinds of format conversions owned by the display conversion function 404 built in the motion picture reproduction monitor 403. Even in a general apparatus handling contents, the transformation applied the contents is limited to some extent.